[TIFF loading] gracefully handle malformed metadata

Previously we would create a TaggedImage with null metadata when metadata
loading fails. Now we create a TaggedImage with an empty JSONObject as
metadata instead, which means that downstream code that assumes the metadata
is non-null will not automatically fail.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14653 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd